1. Field of the Invention
The present invention relates to methods and apparatus for measuring signal timing and, more particularly, to techniques for accurately identifying a direct path signal in the presence of multipath signals.
2. Description of the Related Art
Accurate determination of signal timing is desirable in a wide variety of communication and navigation applications where precise, reliable signal reception is desired. For example, U.S. Pat. Nos. 6,453,168 and 6,801,728, the disclosures of which are incorporated herein by reference in their entireties, disclose state-of-the-art position location and communication systems that provide accurate, reliable three-dimensional position determination of a handheld or portable, spread spectrum communication device within milliseconds without interruption of voice or data communications. Using spread spectrum waveforms and processing techniques, the system is capable of determining position location to an accuracy of less than ten centimeters in a severe multipath channel particularly found in urban indoor environments and provides communications performance commensurate with the modulation and error correction coding used in such environments.
Among techniques employed to determine the position of a mobile communication device is the reception at the mobile communication device of multiple timing signals respectively transmitted from multiple transmitters at different, known locations (e.g., global positioning system (GPS) satellites or ground-based transmitters). By determining the range to each transmitter from the arrival time of the timing signals, the mobile communication device can compute its position using trilateration. When measuring the range to an object or another device, a precise determination of the signal propagation time between the devices must be made. The signal propagation time can be derived by knowing the transmission and reception times of one or more ranging signals traveling along a direct path between the devices. For example, the well-known global positioning system (GPS) relies on measurement of the one-way propagation time of signals sent from each of a set of satellites to a receiving device in order to determine the range to each satellite and the position of the receiving device.
The accuracy and operability of such position location techniques can be severely degraded in the presence of multipath interference caused by a signal traveling from a transmitter to the receiver along plural different paths, including a direct path and multiple, longer paths over which the signal is reflected off objects or other signal-reflective media. Unfortunately, multipath interference can be most severe in some of the very environments in which position location techniques would have their greatest usefulness, such as in urban environments and/or inside buildings, since artificial structures create opportunities for signals to be reflected, thereby causing signals to arrive at the receiver via a number of different paths.
The position determining systems described in the aforementioned patents rely on a two-way, round-trip ranging signal scheme. Specifically, a master mobile communication device transmits outbound ranging signals to plural reference communication devices which respond by transmitting reply ranging signals that indicate the location of the reference radio and the signal turn around time (i.e., the time between reception of the outbound ranging signal and transmission of the reply ranging signal). Upon reception of the reply ranging signal, the master radio determines the signal propagation time, and hence range, by subtracting the turn around time and internal processing delays from the elapsed time between transmission of the outbound ranging signal and the time of arrival of the reply ranging signal. In this manner, the individual radios do not need to be synchronized to a common time reference, thereby eliminating the need for highly accurate system clocks required in conventional time-synchronized systems. The brief ranging messages can be interleaved with voice and data messages in a non-intrusive manner to provide a position determining capability without disruption of voice and data communications.
To provide high accuracy range estimates, the time of arrival (TOA) of the ranging messages are precisely estimated. By performing internal delay calibration, errors caused by difficult-to-predict internal transmitter and receiver delay variations can be minimized. The system uses state-of-the-art spread spectrum chipping rates and bandwidths to reduce multipath interference, taking advantage of existing hardware and software to carrying out a portion of the TOA estimation processing. Frequency diversity can be used to minimize deep fades on the direct path to ensure an accurate TOA range estimate.
The accuracy of the position determined by these systems depends largely on the accuracy with which the receiving devices can determine the time of arrival of the ranging signals traveling along a direct path between the devices. In an environment where multipath interference is significant, it is possible to mistakenly identify a strong multipath signal as the direct path signal. Since a multipath signal travels along an indirect path between the transmitter and receiver, the signal propagation time and, hence, the observed range differ from that of the direct path. In a position determining system relying on precise measurements of direct-path signal propagation time to determine range, erroneously interpreting a multipath signal as the direct path signal can drastically degrade performance. In particular, a multipath signal may result in a severely erroneous range measurement; nevertheless, if the multipath signal has a relatively high signal-to-noise ratio, the erroneous range measurement will be reported to the tracking filter as being highly accurate. Consequently, the filter will be misled into placing a high degree of reliance on a severely erroneous range measurement, thereby degrading the accuracy of the position estimate without the degraded accuracy being immediately known or reported.
In GPS systems, the transmitter/receiver systems attempt to perform time-of-arrival (TOA) determination on the GPS spread-spectrum waveform by correlating the received signal with a replica of the transmitted signal and then finding the time location of the peak magnitude of the correlation. These systems either locate the peak directly or by curve-fitting an ideal correlation function (a triangular pulse) with the actual received signal correlation function. Three recent innovations in receiver technology improve the TOA estimation accuracy of the GPS receiver in a multipath environment. First, narrow correlators use a correlator spacing of a fraction of a chip rather than chip-spaced correlators. This greatly reduces the magnitude of the maximum TOA error in ranging. Second, a multipath estimating delay lock loop (DLL) can be used, which assumes that no more than a certain number (e.g., two) of dominant multipath signals are present. The DLL estimates the amplitude, delay, and phase of each multipath component using maximum likelihood criteria. Each estimated multipath correlation function is then subtracted from the measured correlation function. The remaining direct path correlation function has minimal multipath degradation, and it can be used for accurate TOA estimation. Finally, leading edge curve fitting can be used to match the received signal correlation with an ideal correlation function on the leading edge of the received signal correlation. This minimizes the impact of any delayed multipath signals when computing the TOA, because the multipath has its greatest influence on the trailing edge of the correlation.
A good strategy to improve position accuracy is to increase both bandwidth and signal-to-noise ratio (SNR). Ideally, increasing bandwidth is the best way to improve accuracy because of the inverse square root relationship of improved accuracy with increasing SNR. However, it is not always feasible to obtain a frequency allocation with enough bandwidth to support the desired location determining accuracy, particularly in view of increasing demands on available frequency spectrum.
It would be advantageous in the system described above to boost the weak but desirable signals (e.g., the direct-path signal) for achieving the desired location estimation accuracy. The need to boost the direct-path signal in the pool of multiple received signals is much more evident in the indoor environment where it is not uncommon for the direct path to be tens of dB down from the dominant reflected paths. Inside a building, a direct path from the outside may go through several walls to reach the intended receiver located in a room, and could subsequently be buried by a strong or several strong reflected paths that hit an adjacent building and bounce back through a window to the same receiver. The difference in power between the direct path and the reflected path can be very large. This is one of the challenging problems in indoor ranging.
The aforementioned techniques of using a narrow correlator technique and a multipath estimating delay lock loop can not overcome the tremendous power disadvantage suffered by the direct-path signal in the severe indoor multipath channel mentioned above, which inherently limits the TOA ranging estimate accuracy.
Sending the ranging waveform at different carrier frequency as in Quadrature Multi-Frequency Ranging (QMFR) alone will not solve this particular problem either, because the direct path may again suffer large attenuation as a result of by going through several walls. Frequency diversity, on the other hand, can reduce the probability of the direct-path signal suffering deep fades along with other reflected paths. Nevertheless, there remains a need to improve the capability of accurately detecting the presence and timing of a direct-path in severe multipath environments in a variety of communication and navigation applications.